1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, that records an image on a recording medium by scanning the recording medium with a plurality of beams.
2. Discussion of the Background
In an image forming apparatus, such as for example, a digital copying machine or a printer, it is well known that information is recorded on a recording medium by scanning the recording medium with a single beam. A beam modulated according to an information signal is deflected by a rotating polygon mirror or the like so as to scan a surface of the recording medium in a sub-scanning direction, repeatedly in a main scanning direction. In such an image forming apparatus, the scanning beam from the polygon mirror is detected by a beam detector placed in a position out of a recording area of the recording medium and start of image formation with the scanning beam on the recording medium is controlled according to a beam detect signal from the beam detector.
It is also known that information is recorded on a recording medium by a plurality of scanning beams, which are independently modulated according to individual information and which are simultaneously deflected by a rotating polygon mirror or the like. The plurality of scanning beams scan a surface of the recording medium in a sub-scanning direction, repeatedly in a main scanning direction. In such an image forming apparatus using a plurality of beams, the beams are respectively detected by a plurality of beam detectors placed in a position out of a recording area of the recording medium, and start of image formation on the recording medium with each scanning beam is controlled according to a corresponding beam detect signal from the plurality of beam detectors.
Japanese Laid-open Patent Publication No. Sho57-8887 describes a method of controlling a start position for writing image information on a recording medium in an image forming apparatus which writes information by scanning the recording medium with a plurality of beams. In this method, a timing signal for each beam is generated by comparing a beam detect signal to a threshold level, and a start position to start writing image information is controlled according to the generated timing signal.
Japanese Laid-open Patent Publication No. Hei 6-206343 describes another control method for an image forming apparatus in which information is recorded by scanning a recording medium with a plurality of beams from semiconductor lasers. In this method, a synchronizing signal for each beam is generated by controlling on/off state of the semiconductor lasers by using a clock cycle which is shorter than a picture element clock cycle, and start of image formation with each beam is controlled according to the synchronizing signal generated for each beam.
In an image forming apparatus in which information is recorded by scanning a recording medium with a single beam, when recording the information at high speed or at high resolution, because a beam modulated according to an information signal is deflected by a rotating polygon mirror or the like so as to scan a surface of the recording medium in a sub-scanning direction, repeatedly in a main scanning direction, it is necessary to increase the transmission speed of the information signal for modulating the beam.
Also, because the scanning speed for the main scanning direction must be increased for accomplishing high speed or high resolution recording, when a rotating polygon mirror is used for deflecting the beam, the rotating speed of the polygon mirror must be increased, for example, to several tens of thousands rpm, which is hard to realize due to structural limitations.
The above-described image forming apparatus using a plurality of scanning beams has been proposed to solve the difficulties in accomplishing high speed or high resolution recording with a single beam. In such an image forming apparatus using a plurality of scanning beams, it is necessary to dispose the plurality of beams at intervals that are is substantially the same as those of picture elements to be formed on a surface of a recording medium. However, the interval of light emitting points of a semiconductor laser (which is generally used as a light source for emitting the plurality of beams) is generally broader than the interval of picture elements for recording information with a generally desired writing resolution of 600 dpi. Therefore, as illustrated in FIG. 5, when a light emitting points interval P1 is greater than a picture elements interval Ps, the beam interval on the recording medium is made the same as the picture elements interval Ps by slanting the arrangement of the light emitting points for the beams B1-B4 by a predetermined angle relative to a line L-L' which is perpendicular to the main scanning direction SL.
However, in a multi-beam optical system in which a plurality of beams are disposed closely to each other as described above for writing information at high resolution, when the writing resolution is, for example, 600 dpi, the beams interval Ps must be adjusted to about 42.3 .mu.m. This makes it hard to dispose a plurality of detecting elements on a scanning optical path to detect each beam. Therefore, generally, the plurality of beams scan a single detecting element, and a detect signal for each beam is generated by the single detecting element in a manner as illustrated in FIG. 6. In order to align the image formation start point of each beam with each other beam in a precise manner on a recording medium, the plurality of beam detect signals from the single beam detecting element, corresponding to the plurality of beams, respectively, must be separated from each other in a precise manner, because the image formation start position of each beam modulated according to an information signal must be determined based upon the beam detect signal for each beam.
In the writing start position control method described in JP No. Sho 57-8887, a timing signal for each beam is generated by comparing a beam detect signal to a beam detect device with a threshold level, and controlling the writing start position according to the timing signal. However, because the threshold level for the detect signal is set with an analogue value, generation of the timing signal tends to be influenced by variation of the power of an electric source and variation of the power for each beam, and thereby accuracy in the writing start position is decreased.
In the image forming apparatus described in JP No. Hei 6-206343, a synchronizing signal for each beam is generated by controlling the on/off state of a semiconductor laser by using a clock cycle which is shorter than a clock cycle for picture elements, and start of image formation with each beam is controlled according to the synchronizing signal for each beam. Therefore, high frequency oscillator is necessary for generating the clock for controlling on/off of the semiconductor laser, and thereby a control circuit must be configured with elements suitable for a high frequency, such as a flip-flop type circuit. This increases the cost of the apparatus and also causes a problem of large electromagnetic interference waves.